Fungus Among us

Somehow I managed to get fungus inside my day-to-day DSLR lens. This lens (a Sigma 18-50mm f3.5-5.6 DC lens) cost me only $37 with shipping via eBay, and having it cleaned professionally would cost more than the lens itself. I decided to clean it myself!

It took me about an hour to open it up and clean it (I didn't want to have spare screws when I was finished), but I eventually got the fungus off the inside lens element with isopropyl alcohol. There appears to be some small amount of etching left on the element (fungus secretes hydrofluoric acid), but it doesn't seem to be visible in photos taken with the lens.

Unfortunately, after cleaning and reassembling the lens, my camera showed the aperture go up to 5.0 at the 50mm end (it should be 5.6 at 50mm). The lens thought it was only at 37mm, according to the EXIF data in the JPEG files. I probably bent one of the electrical contacts around the inside of the zoom ring which tells the lens and camera what the current focal length is. For those technical geeks (like myself), there are five electrical contact strips running around the lens underneath the zoom ring, and they appear to be encoded using Gray coding; the contacts on the zoom ring make contact with those strips, which in turn tells the lens the position of the ring. Gray coding is pretty neat and useful that way. (Incidentally, five tracks provides up to 32 combinations, and there are 33 different focal lengths (50-18+1=33). I wonder which focal length(s) are not represented by the lens.)

I took the lens apart a second time (though not as far as the first time), and I straightened out the contacts. After reassembling again and testing again, the focal length is being reported correctly! Woot!

I just hope that I don't get fungus in this lens again. :/